abaldacci,
Since we drive from nearly the same place as you, I'll tell you what we do:
We stay off-site the first night and last night. We then get up early and go to a park, staying until after lunch. During our "break," we check in at the resort. Then, our last day, we check out, then spend the entire day at the parks, usually closing whatever park is open latest before driving an hour or two toward home. We always ask for a late checkout last day, and sometimes get late enough to take a break in the room.
We also eat breakfast in the room every day except one, keep a cooler with soft drinks, water and milk, and travel with a boatload of snacks for the room and the car. And, or course, we drive. I'm perfectly willing to make these "sacrifices," but not to stay off-site. We go to Disney to totally escape from the real world and you can't do that if you leave every evening.
Staying off the first and last nights will save you $ but not $500. If you had planned to fly or eat breakfast out, this WILL save you that much $ and more.